It's not difficult
You start the game, look in the bottom left corner, and it should tell you which version you have. It will either say:
1.0
1.1
1.3
If you have 1.1 or 1.3, you should have the mission editor. If you have 1.0, you need to dowload the mission editor at http://www.shipsim.com/downloads/patches.php or better still, the whole 1.3 patch
When it has finished downloading, you will have the editor on your desktop